Linux Capability

This article shows configuration and dependent gem installation instruction for enabling Linux capability module on Fluentd core.


  • gcc and make etc. for building C extension sources

  • libcap-ng package and its depelopment package

    • libcap-ng-dev on Debian GNU/Linux and Ubuntu

    • libcap-ng-devel on CentOS 7/8, Fedora 33, AmazonLinux 2

  • pkg-config package for linking libcap-ng library

  • Ruby and its development packages

    • ruby-dev on Debian GNU/Linux and Ubuntu

    • ruby-devel on CentOS 7/8, Fedora 33, AmazonLinux 2

  • Fluentd v1.12 or later

Install capability handling gem

Fluentd uses capng_c gem to handle Linux capability.

So, Add this line to your Fluentd' or td-agent's Gemfile:

gem 'capng_c'

And then execute:

$ bundle

Or install it yourself as for Fluentd:

$ fluent-gem install capng_c

Or install it yourself as for td-agent:

$ td-agent-gem install capng_c

Note: capng_c uses pkg-config to link libcap-ng library. If you couldn't handle Linux capability with capng_c installation, please confirm pgk-config package is installed on your box.

Capability handling on in_tail

Currently, in_tail which is the one of the Fluentd core plugin handles the following Linux capabilities:

  • CAP_DAC_READ_SEARCH (:dac_read_search on in_tail code.)

  • CAP_DAC_OVERRIDE (:dac_override on in_tail code.)

Set up cap_dac_read_search or cap_dac_override to using Ruby executable:

$ sudo fluent-cap-ctl --add dac_read_search [-f /path/to/bin/ruby]
Updating dac_read_search done.
Adding dac_read_search done.


$ sudo fluent-cap-ctl --add dac_override [-f /path/to/bin/ruby]
Updating dac_override done.
Adding dac_override done.

Example setting up capability for rbenv-ed Ruby

$ sudo fluent-cap-ctl --add "dac_override,cap_dac_read_search" -f $(rbenv prefix)/bin/ruby
Updating dac_read_search,dac_override done.
Adding dac_read_search,dac_override done.
$ fluent-cap-ctl --get -f $(rbenv prefix)/bin/ruby
Capabilities in '/home/fluentd/.rbenv/versions/2.6.3/bin/ruby',
Effective: dac_override, dac_read_search
Inheritable: dac_override, dac_read_search
Permitted: dac_override, dac_read_search

Actual Example for Linux capability handling in in_tail

When adding cap_dac_override (partial privileges for rw file) and cap_dac_read_search (partial privileges for read only), Fluentd/td-agent can handle to read 640 permission files such as /var/log/syslog:

$ ls -lh /var/log/syslog
-rw-r----- 1 syslog adm 29K Nov 5 14:35 /var/log/syslog

This file cannot read form ordinal users:

$ cat /var/log/syslog
cat: /var/log/syslog: Permission denied

Attach dac_read_search for using Ruby executable binary:

$ sudo fluent-cap-ctl --add dac_read_search [-f /path/to/bin/ruby]
Updating dac_read_search done.
Adding dac_read_search done.
$ fluent-cap-ctl --get [-f /path/to/bin/ruby]
Capabilities in '/path/to/bin/ruby',
Effective: dac_read_search
Inheritable: dac_read_search
Permitted: dac_read_search

And prepare the following configuration:

@type tail
path /var/log/syslog
pos_file /var/run/fluentd/log/syslog_test_with_capability.pos
tag test
rotate_wait 5
read_from_head true
refresh_interval 60
@type syslog
<match test>
@type stdout

Make and change ownership directory:

$ sudo mkdir /var/run/fluentd
$ sudo chown `whoami` /var/run/fluentd

Then, run as ordinal user with cap_dac_read_search capability attached Ruby:

$ bundle exec fluentd -c in_tail_camouflage_permission.conf
2020-11-05 14:47:57 +0900 [info]: parsing config file is succeeded path="example/in_tail.conf"
2020-11-05 14:47:57 +0900 [info]: gem 'fluentd' version '1.12.0'
2020-11-05 14:47:57 +0900 [info]: gem 'fluent-plugin-systemd' version '1.0.2'
2020-11-05 14:47:57 +0900 [info]: using configuration file: <ROOT>
@type tail
path "/var/log/syslog"
pos_file "/var/run/fluentd/log/syslog_test_with_capability.pos"
tag "test"
rotate_wait 5
read_from_head true
refresh_interval 60
@type "syslog"
<match test>
@type stdout
2020-11-05 14:47:57 +0900 [info]: starting fluentd-1.12.0 pid=12109 ruby="2.6.3"
2020-11-05 14:47:57 +0900 [info]: spawn command to main: cmdline=["/home/fluentd/.rbenv/versions/2.6.3/bin/ruby", "-rbundler/setup", "-Eascii-8bit:ascii-8bit", "/home/fluentd/work/fluentd/vendor/bundle/ruby/2.6.0/bin/fluentd", "-c", "example/in_tail.conf", "--under-supervisor"]
2020-11-05 14:47:58 +0900 [info]: adding match pattern="test" type="stdout"
2020-11-05 14:47:58 +0900 [info]: adding source type="tail"
2020-11-05 14:47:58 +0900 [info]: #0 starting fluentd worker pid=12143 ppid=12109 worker=0
2020-11-05 14:47:58 +0900 [info]: #0 following tail of /var/log/syslog
2020-11-05 09:53:11.000000000 +0900 test: {"host":"fluentd-testing","ident":"anacron","pid":"22613","message":"Job `cron.daily' terminated"}
2020-11-05 09:53:11.000000000 +0900 test: {"host":"fluentd-testing","ident":"anacron","pid":"22613","message":"Normal exit (1 job run)"}
2020-11-05 09:55:01.000000000 +0900 test: {"host":"fluentd-testing","ident":"CRON","pid":"24610","message":"(root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)"}

Fluentd which is running on ordinal user does not complain as Permission denied. Users can retrieve root files' contents on non-root process, yay!

If this article is incorrect or outdated, or omits critical information, please let us know. Fluentd is an open-source project under Cloud Native Computing Foundation (CNCF). All components are available under the Apache 2 License.